Bonded non-woven fibrous articles, such as batts and shaped articles such as cushions, toys and fabrics, have previously been made from an air-laid or wet-laid blend of low melting fusible fibers with higher melting or nonfusible fibers that are bonded and become the load-bearing structural elements (or at least supply a significant portion thereof) in the eventual bonded structure. The fusible fibers are generally referred to herein as binder fibers, but are also referred to as a second fiber constituent of the blends. The higher melting fibers are generally referred to herein as load-bearing fibers, or as a first constituent of the blends. In the resulting bonded articles, such bonded fibers have a relatively high temperature resistance in contrast to a relatively low flow temperature for the bonding agent that bonds the fibers. The term "through bonding" is used to indicate what is generally desired in relation to such bonded articles, namely sufficient bonding throughout its depth so the batt acts as integral unit, i.e. so as to avoid separation between any individual layers forming the original batts. Bonding derived from spraying (and then activating) a liquid resin binder onto a batt has not generally provided such through bonding, because spraying a liquid binder onto the surface(s) of a preformed batt does not generally enable the binder resin to penetrate throughout the batt.
Through-bonded batts have been bonded hitherto by conventional means of heating, such as conduction, convection, or radiation. I have noted that such bonding of these batts by such conventional means has been a slow and timeconsuming operation. The batts are good insulators, i.e. their conductivity is low, natural convection is difficult and infrared radiation cannot penetrate the fibers. Other problems have been caused by the slow rate of heat transfer, which can lead to overheating and thermal degradation of outside fibers, and/or the inside of the batt may not even get hot enough to activate the binder fibers properly.
Street U.S. Pat. Nos 4,668,562 and 4,753,693 and Wm. T. Burnett (Brooks) WO 88/00258, discuss and indicate some of the problems encountered heretofore. Thus, forced convection can be used to speed up the heating, i.e. hot air can be forced through the structure to be bonded. But forced convection can be used to bond only certain structures such as lend themselves to air suction or blowing. Also, the force exerted by the air flow pulls the fibers close to each other, i.e. densifies the batt, so lighter weight bonded structures cannot be prepared by this technique.
Thus, prior techniques have not only been slower than would be desirable, but have led (sometimes) to undesirable structural characteristics. A problem, for example, in some high density batts has been that the use of heated air has created non-uniform density, with a higher density at the bottom of the batts. I believe that the main cause has been that the load-bearing fibers have been heated to the same temperature as the binder fibers and, at such temperature, are close to their glass transition temperature. The load-bearing fibers lose some of their desirable characteristics at these temperatures. For instance, these fibers near the bottom of the batt have given way under the weight of the upper portion of the batt. Another deficiency of many prior high density, hot air-bonded blocks has been that the middle has been less bonded than the surface, because the surface has been more accessible to the heated air. Usually, for products such as cushions or mattress cores, it is desired to have a firmer (more bonded) center and softer outside layers. In hot air-bonding, just the opposite has happened.
So, an important objective of the present invention has been to improve the speed of the thermal bonding process over what has previously been done commercially. Another objective has been to overcome some of the limitations that may have been inherent in various prior commercial bonding techniques. Cost-effectiveness is always an important objective for any commercial operation.
I have succeeded by using heat generated internally within the fibrous structure by an oscillating electric field, referred to generally herein as dielectric heating, and produced by electromagnetic radiation (EMR). Others may have been discouraged because existing commercially-marketed fibers, in general, and polyester textile fibers, in particular, have not generated enough energy in an oscillating field. Indeed, polyesters in general are good insulators and have low dielectric or inductive loss. This property is why polyesters are useful in capacitors.
It has been suggested (e.g. in U.S. Pat. Nos. 3,535,481 (Korb) and U.S. Pat. No. 4,003,840 (Ishino) and in GB 2,196,343) to introduce extraneous particles, such as ferrometallic or other conductive materials, into the structure and generate heat inductively, but this technique has not proved satisfactory for my purposes.
Paul, in U.S. Pat. No. 4,401,708 claims a method of bonding using microwave energy and a polar solvent (such as trichloroacetic acid) to produce non-woven fabrics, especially for use as carpet backing. This method may be advantageous for the special end-use indicated, but, for most purposes, would present practical problems of control, e.g., in applying the solvent to the appropriate locations (for instance to all the fibers), at the time desired (which would usually be just before bonding), without excessive degradation by the solvents upon prolonged exposure to fibers, and uniformly (so as to heat the fibers uniformly, if desired), or selectively (when it would be desired not to heat or otherwise affect some fibers). Also, such solvents do weaken the fibers, as indicated. Such solvents can also be toxic. So there would be several disadvantages in trying Paul's approach to solve my problem.